The present invention relates generally to turbines, particularly to land-based gas turbines for power generation employing closed circuit steam-cooling paths for cooling the hot gas components and particularly relates to a rotor bore and turbine wheel/spacer flow circuit having a thermal medium in heat exchange relation with the wheel/spacers.
Steam-cooling of hot gas path components, for example, the buckets of a gas turbine has been proposed in the past and found efficacious in land-based power generating plants. While gas turbines are typically air-cooled, for example, jet turbines employ compressor discharge air for cooling the hot gas components, steam-cooling is more efficient in that the losses associated with the use of steam as a coolant are not as great as the losses realized by extracting compressor bleed air for cooling purposes. Also, in combined cycle operations, steam-cooling is particularly advantageous because the heat energy imparted to the steam as it cools the gas turbine components is recovered as useful work in driving the steam turbine in the combined cycle operation.
In a proposed gas turbine design of assignee hereof, a steam-cooling circuit is employed during normal operation to cool the hot gas path components. The steam-cooling circuit includes an aft axial bore tube assembly for supplying steam forwardly through the rotor past the aft rotor bearing. The flow continues radially outward through linear tubes, then axially along the rim of the rotor at circumferentially spaced locations. The cooling steam is also returned along the outer rim of the rotor then flowing radially inwardly and then flowing axially through the bore tube assembly. However, at startup, the steam-cooling circuit can be used as an air-cooling circuit with cooling air circulating through the steam flowpaths used during normal operation. At about 10% load, the air-cooling circuit is switched over to the steam circuit, which would then be used for normal operations.
It will be appreciated that in rotor construction, the wheels and spacers are secured to one another by a plurality of axially extending bolts. Rabbeted joints are provided between the wheels and the spacers. Differential heating of the wheels and spacers, as well as radial thermal gradients across the rabbeted joints, however, cause significant rotor bore stresses and deflections which tend to open up the rabbeted joints. It is also noted that the thermal conditions of the rotor including the wheels and spacers are different at startup, steady-state operation and turbine shutdown. Consequently, there is a need to provide a heat exchange circuit which will accommodate these different requirements at the different stages of turbine operation, accommodate these thermal gradients and preclude a wheel and spacer thermal response which would open up the rabbeted joints.
In accordance with a preferred embodiment of the present invention, there is provided a flow circuit for a thermal medium, preferably air, which may pre-warm and heat up the wheels and spacers during startup of the turbine and cool the wheels and spacers during steady-state turbine operation whereby rotor deflections are significantly controlled and the tendency of the rabbeted joints to open in response to thermal gradients is minimized or eliminated. To accomplish the foregoing, compressor discharge air is supplied to the rotor bore forwardly of the first stage for flow axially along the rotor bore and into the cavities between the wheels and spacers. At the juncture of the wheels and spacers and particularly at the rabbeted joints, channels or slots are formed in those joints at circumferentially spaced positions about the rotor to channel the thermal medium radially outwardly and ultimately for flow into the hot gas path.
It will be appreciated that during startup of the turbine, the compressor discharge air pre-warms and heats up the wheels and spacers as the flow passes radially along the axially registering faces of the wheels and spacers. By heating the wheels and spacers at startup, the thermal mismatch or gradient between the rim of the rotor and the heavy mass of the wheels and spacers radially inwardly of the rotor is diminished, thereby minimizing the stress on the rabbeted joints. During steady-state operation, however, compressor discharge air cools the wheels and spacers to reduce the thermal gradient with the outer rim and hot gas components of the rotor which are steam-cooled. Thus, during steady-state operation, rotor bore stress is likewise minimized, reducing or eliminating the tendency of the rabbeted joints to open up.
In a preferred embodiment according to the present invention, there is provided a rotor having an axis and including a plurality of turbine wheels and spacers disposed alternately between the wheels, the wheels and spacers defining cavities therebetween, the turbine wheels mounting buckets for disposition in a hot gas path of the turbine, the wheels and spacers being secured to one another and defining a passage along the axis of the rotor for receiving compressor discharge air in communication with the cavities, the wheels and spacers having axially and circumferentially extending radially confronting flanges forming a rabbeted joint therebetween, a first set of a plurality of circumferentially spaced slots carried by the wheel flanges and a second set of a plurality of circumferentially spaced slots carried by the spacer flanges, each wheel and spacer in axial confronting relation with one another being rotationally aligned to enable the flow of the compressor discharge air from the cavity through the aligned slots into the hot gas stream.
In a further preferred embodiment according to the present invention, there is provided in a gas turbine having a plurality of turbine wheels and spacers disposed alternately between the wheels, the wheels and spacers being secured one to the other to form a turbine rotor having a central axial passage, the wheels and spacers defining cavities therebetween, the wheels mounting buckets for disposition in a hot gas path of the turbine, a method of operating the turbine comprising the step of, during startup, supplying compressor discharge air along the axial passage and into the cavities between the wheels and spacers to heat the wheels and spacers.
In a still further preferred embodiment according to the present invention, there is provided in a gas turbine having a plurality of turbine wheels and spacers disposed alternately between the wheels, the wheels and spacers being secured one to the other to form a turbine rotor having a central axial passage, the wheels and spacers defining cavities therebetween, the wheels mounting buckets for disposition in a hot gas path of the turbine, a method of operating the turbine comprising the step of, during steady-state operation of the turbine, supplying compressor discharge air along the axial passage and into the cavities between the wheels and spacers to cool the wheels and spacers.